What the heck are percentiles, anyway?

The Pediatric Insider

© 2011 Roy Benaroch, MD

In pediatrics, our patients are growing targets. There’s no “best” weight or “correct” blood pressure—there’s averages and ranges that depend on things like a child’s age and sex. Since there’s no way we could possibly memorize all of the normals at every age, we rely on “percentiles.”

Talking in percentiles doesn’t always make sense to parents. I blame number grades in school, where the closer to 100% your child gets, the better the grade. “I scored a 97!” is great. Having a BMI (body mass index) percentile of 97%– that’s not so great.

A percentile is a way of comparing your child to kids of similar age and sex. If your son’s height percentile is 40%, that means he’d be number 40 in a line of boys of his exact age if they were lined up in height order. A percentile at or near 50% is about average, and anything between about 25-75 percentile is close enough to be considered average.

In most areas of health, average equals good. It’s the outliers, the ones with the highest blood pressure or the lowest blood counts, that we worry about.

Percentiles are especially useful when we look at growth and weight. Most children grow along about the same percentile range from age 2 through puberty—so if after two the percentile is changing much, something might be going wrong with growth (before two, there is a lot of percentile shifting as children move towards their expected growth pattern.)

A person’s overall “chubbiness” is usually expressed numerically as a BMI, or body mass index. In adults, a BMI of 25 is usually considered overweight; over 30 is obese. In kids, we rely on the BMI percentile—over 85% is overweight, over 95% is obese. From year to year, the BMI number will change, but the percentile should not vary very much.

Another thing about percentiles: in the middle of the pack, a very small change in a number will lead to an exaggerated change the percentile number that really isn’t very meaningful. For instance, a  9 year old boy who weighs 79 pounds is at the 50th percentile. If he gains 3 pounds, that takes him to the 60th percentile. But a change from 85 percentile to 95 percentile in the same boy would mean he’s gained 15 extra pounds. Percentile changes in the 25-75th percentile range usually don’t mean there’s been a big change in absolute numbers, but percentile changes of only a few points way at the top of bottom of the percentile range can mean a big shift has occurred.

If you’re concerned about your child’s growth or weight, ask your pediatrician to review the growth chart and show you how the percentiles have trended over the years. For most kids, a nice stable percentile curve means that their overall health is good—even if the percentile isn’t right the middle. But a child who’s percentile is very far from average (especially those with BMIs higher than 85-95 percentile), may have significant health risks that ought to be addressed.

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9 Comments on “What the heck are percentiles, anyway?”

  1. TwinMom Says:

    Thanks for this article!

    My three year old twins have BMI’s in the 85 and 90th percentiles. I was a little worried about this, but two pediatricians said that unless the parents are overweight they generally don’t worry about BMI in small children. The docs also looked at the kids and said that they “don’t look fat” (which is true). What’s your thought about BMI in three year olds?


  2. Dr. Roy Says:

    TwinMom, BMIs are valid measurements at age 3. Though I wouldn’t ever put a 3 year old on a “diet” or encourage weight loss, I would typically notice a BMI over 85%ile, and review healthy lifestyle decisions about minimizing screen time and sweetened beverages. I’d track the BMI as well, to make sure it wasn’t further increasing. A few simple lifestyle tweaks at age 3 is a whole lot easier than a big weight loss program in a teenager.


  3. TwinMom Says:

    Thanks, Dr. Roy!

    The thing is that I think my kids have a fairly healthy lifestyle. They only drink non-fat milk, and water (plus about 3 oz of juice once a day in the morning). We don’t do sweets except for special ocassions. I try to limit them to an hour of screen time a day – though now that they aren’t napping, sometimes it’s more like 1.5 hours. I cook most meals and I despise fast food, so they’ve never been to a fast food restaurant. They are basically moving non-stop from the moment they get up until they go to bed.

    I do have to say that they are frequently ravenously hungry and will sometimes eat large quantities of food. I hate to tell them – “no, you can’t have any more to eat” – because they get seriously cranky and whiny if they are hungry. Plus, it seems like a good way to set them up for food issues later in life.

    I was very chubby as a child until I hit about 6 and then got very skinny until I hit puberty. I’m now a healthy, normal weight adult.

    Do you think this could just be genetic and that it will all even out in the end? Or, do you think I should cut out things like cheese from their diet. Life kind of doesn’t seem worth living without cheese. 🙂


  4. Dr. Roy Says:

    No cheese? No way!

    I wouldn’t make any big changes. Maybe try to slow meals a bit– if kids eat very fast, they’ll eat far more than if they slow down (adults too!) Encourage more conversation and sips of water at mealtimes.

    Some math to illustrate what little habits can do: 3 oz of juice a day is about 45 calories a day, or about 16,500 calories a year. Making some huge assumptions about basal metabolism, let’s estimate 1 pound of extra body fat is created by 3500 extra calories (BTW, that’s not a bad approximation for an adult.) That 3 oz of juice every day will lead to 4.5 extra pounds a year. And a 3 year old is probably supposed to grow only 6 pounds or so a year anyway!

    Little tiny changes in daily habits can really lead to big changes in weight.


  5. Jamie Says:

    How do percentiles work in a population that’s becoming more obese on average over time? Are there kids now who are being told they are a healthy weight when in fact they are just an averagely unhealthy weight?


  6. Dr. Roy Says:

    Jamie, the data that creates the reference percentiles doesn’t change. Most growth charts use data from the early 1970’s. So right now, there are far more than 15% of us that are above the 85 percentile.


  7. Rachel Abbott Says:

    My 3.5 year old is .51%. She has had ftt since she was 1.5 years. Is this something for much concern? Me and my husband are regular.


  8. crac Says:

    @dr roy,
    “Most growth charts use data from the early 1970′s.”

    Then that should be the first thing you says, because that directly contradicts what you said elsewhere :

    “If your son’s height percentile is 40%, that means he’d be number 40 in a line of boys of his exact age if they were lined up in height order.”

    And that’s also assuming the “average” was healthier in the 70s.

    So many assumptions, I guess you should start by taking them all out of the way.


  9. roxanne Says:

    I understand percentages bit my question is what if a child is 95 percentile for weight, but 90th percentile for height wouldn’t that be close to a correct weight than say they were in the 45th percentile for weight but only in the. 5Th percentile for height…..even if both looked the same as far as being fit looking or both had the same BMI the taller one would automaticallt be higher in weight percentile….seems like the closer the percentile numbers are the more that person would be weighing what they are supposed to weigh. ……does that question make sense?


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